Cellulose ethers have been used in various fields such as industrial fields, food fields and pharmaceutical fields. Reaction slurries typically contain neutralization salts, methanol, alkylene glycols and other by-products generated during the production of cellulose ethers. Some of these by-products interfere with characteristics of the cellulose ether and are substances harmful to human health. Thus, the by-products are required to be removed.
When a cellulose ether is produced, it is difficult to suppress one or more side reactions of an etherifying agent, and after the etherification reaction is over, it is also required to remove a large amount of neutralization salt or salts derived from an alkali used for alkalization of cellulose. The side reactions in the etherification step of the obtained alkali cellulose are exemplified by the hydrolysis of an etherifying agent or the competitive substitution reaction of OH groups of the by-product derived from an etherifying agent. For example, JP 52-25887A discloses a process in which a cellulose ether is continuously produced and, after completion of the reaction, an unreacted etherifying agent and some of the by-product, dimethyl ether, are recovered and reused in the etherification step. The component to be reused, which is a low boiling component in the system after completion of the reaction, is isolated with a partial condenser and returned into a reaction slurry in the reaction system, the reaction slurry being in a process of subjecting the crude product to hydrothermal treatment. The component not to be reused, which is the residue having the low boiling component removed, is circulated into a reaction slurry in the washing system.